答案：A

题目分析：考查虚拟语气：句意：要不是因为雾霾天气，那时北京的交通事故会更少，从Had it not been for the haze（霾）weather phenomenon,可知是和过去相反的虚拟条件句，相当于：If it had not been for the haze（霾）weather phenomenon,所以主句也是和过去相反的虚拟语气，用would have done，选A。